I've read a lot of threads from people who have some kind of malfunction with the engine of their motorcycle.

As a service, I have come up with a general check list for these brave shade-tree mechanics.

First of all, all internal combustion engines (not diesel) depend upon the correct function of three critical processes.

1. Fuel delivery
2. Air delivery
3. Ignition delivery (in diesels, compression = heat = ignition = no spark plugs)

Air and fuel are mixed in a ratio of 12.5:1 - 13.3:1 for most applications. Computers control the delivery of the electric spark at the correct time in the piston's travel.

An engine will not operate properly if these criteria are not met.

Rough idle or poor performance could be due to any one of these critical processes not being delivered correctly.

Check spark delivery first. Misfires due to shorts in the ignition are easy to check for. Pull a plug wire and check its resistance with a multimeter/ohmmeter. A high reading or infinity will indicate a faulty wire.
Check the plugs themselves. Normal spark plugs will have a golden brown look to them (like fresh biscuits). Black, greasy plugs are fouled due to oil contamination (check gaskets). White, chalky plugs are running in a lean cylinder (check fuel and air delivery).

Fuel delivery problems are most likely associated with maladjusted carburetors, faulty throttle position sensor, clogged fuel filter, clogged fuel injector, fuel line rupture, or vacuum problems.

Air delivery problems usually stem from a clogged air filter or blocked air box. Squirrels are known to nest in airboxes of motorcycles that are not ridden frequently.


Please feel free to add to this list as it is most certainly not comprehensive.